Jan 6 (Reuters) - Currency speculators raised the number of net short bets on the Mexican peso this week, again increasing the number of contract bets that the peso will fall after a small decrease last week.

The value of net short positions against the peso rose to $1.55 billion dollars, data from the Commodity Futures Trading Commission released on Friday showed.

Net short positions against the peso had risen for six straight weeks until the week of Dec. 20 following the surprise election of Donald Trump as U.S. president. Trump’s promises to rewrite the North American Free Trade Agreement and take a hardline stance on immigration have sent the value of Mexico’s currency to all-time lows since the election.

The pause in adding net peso shorts in December was the result of Trump’s relative quiet after winning the election, said Alfonso Esparza, currency strategist at Oanda in Toronto.

“It was the unknown regarding Trump - how is he going to behave and is it going to change?” he said. “But now we’ve seen starting in January that he’s back on, so all of that is being reflected in the expectations of investors and what’s going to happen next. So, peso shorts are still favored.”

This week the peso’s value touched a fresh record low, falling to 21.619 pesos per dollar.

Ford Motor Co announced on Tuesday it was scrapping a proposed new $1.6-billion plant in Mexico and choosing to instead build it in Michigan. Despite the company’s insistence that the change was the result of sagging demand for small cars in North America, it stoked fears that Trump’s protectionist rhetoric could indeed pose serious threat to the Mexican economy.

Trump said during the election campaign that if elected he would not allow Ford to open the new plant in Mexico.

Net short contracts on the peso fell during the last week of 2016, breaking the six-week streak, but again rose this week, hovering just above their highest since October. Speculators took out more than 87,000 short contracts on the peso and just over 22,000 long contracts during the week that ended Jan. 3.

To be short a currency is to bet it will decline in value, while being long is a view its value will rise. (Reporting by Dion Rabouin; editing by Grant McCool and Lisa Shumaker)